Our group has developed a model for lipid transport between blood and the interior of cells in tissues by lateral movement in an interfacial continuum of the outer leaflets of various cell membranes. Our studies in white adipose tissue and heart, reported last year, gave morphological evidence for an interfacial continuum extending from the luminal surface of capillaries to lipid droplets and mitochondria in parenchymal cells. Recent studies in brown adipose tissue and kidney demonstrate that the membrane of endoplasmic reticulum is continuous with the outer mitochondrial membrane. Thus, fatty acids formed by action of tissue (hormone-sensitive) lipase on intracellular triacylglycerol can enter the continuum at sites of lipolysis in endoplasmic reticulum and move in the continuum to the outer mitochondrial membrane where they can be activated for transfer to the inner matrix for oxidation. Phase microscopic and radiochemical analyses showed that ionization of long chain fatty acids at pH 7.4 to 9.6 causes rapid enlargement of the oil-water interface and very little loss of fatty acids to aqueous medium, whereas at pH 11 to 12 the rate of growth of the interface is much faster and some of the fatty acids are lost, many as strands, to the aqueous perifusate.